This invention pertains to the art of bearings and more particularly to bearings of the type having an oil film or layer disposed between the bearing surface and a journaled member.
The invention is particularly applicable to halfshell bearings used in internal combustion engines in cooperation with crankshafts and connecting rods and will be described with particular reference thereto. However, it will be appreciated by those skilled in the art that the invention has broader applications and may be used in many other environments of this general type.
As is known, internal combustion engine main and connection rod bearings are subjected to repetitive loads which can ultimately fatigue the bearing alloy if it does not have sufficient strength for the particular applications involved. Metallurgical advances through the years have produced progressively stronger alloys, however, those metallurgical alterations and advances which have thus far been made appear to have reached the limit insofar as improving or increasing fatigue resistance. The primary reason for this is that in order to make an alloy stronger, it must be made harder or alloyed with higher percentages of elements which are not, themselves, good bearing materials. Bearings constructed from bearing materials which are made stronger by either of the two above noted means will, while theoretically having a higher fatique strength, have a much greater tendency to seize during operation and will invariably fail from such seizure.
Typical internal combustion engine main and connecting rod bearings are comprised of halfshells or semi-circular arrangements constructed from conventional bearing materials and are installed within the engines in a manner well known in the art. Because these particular engine components are received in what amounts to continuously circulated oil bath, there is a clearance area provided between the outer peripheral surface of the journaled member or crankshaft and the inner peripheral surface of the bearing. This arrangement not only facilitates relative rotation between the parts in the proper manner during normal engine operation but more importantly, facilitates the creation of an oil film therebetween which acts as a load supporting medium during engine operation. The hydrodynamic wedge effect which causes the shaft or journaled member to float on an oil film is a result of there being a difference between shaft and bearing radii at the associated areas thereof.
For some period of time, it was believed in the industry that fatigue of a bearing alloy could be expressed in terms of maximum unit load (peak load divided by projected bearing area, i.e., length x shaft diameter). However, this belief or concept did not satisfactorily explain why bearings with the same area, but different lengths and diameters, exhibited different fatigue lives under identical loading. Modern analytical methods, specifically the journal orbit analysis, have now explained this phenomenon. Basically, geometric factors such as bearing length, diameter and clearance affect the peak pressure developed in the load supporting oil film. Those bearings exposed to higher pressures fatigue sooner or to a greater extent in the same amount of time as bearings exposed to lower pressures.
Through the use of bearing orbit analysis techniques, it can be shown that the peak oil film pressure developed in a bearing increases in an essentially linear fashion with increased bearing clearance. Thus, a seemingly ready answer for improving fatigue strength for these bearings is to reduce the oil film pressures by simply reducing the bearing clearance. However, a simple reduction in such clearance is not generally preferred since it would reduce the "slop" between the bearings and journaled member and thus be more sensitive to misalignments. Moreover, such a reduction in clearance would also cause the bearing to run hotter than one of normal clearance and it could, therefore, burn up during normal use.
The subject invention contemplates a new structural arrangement which overcomes all of the above referred to problems and others and provides a new bearing structure which is simple in design, provides improved fatigue resistance over conventional bearing structures and which is readily adaptable to use in a number of environments.